[DTrace-devel] [PATCH 30/61] Add a block of zeroes to use to initialize BPF maps

[eugene.loh at oracle.com]

From: Eugene Loh <eugene.loh at oracle.com>

There are cases in BPF code where we want to read a contiguous block of
zeroes.  Since the block is read-only, it can be shared among all CPUs and
among multiple purposes to save BPF map space (locked memory).  Create such
a block.

The block might as well reside in an already existing BPF map to consolidate
allocations.  The BPF_MAP_TYPE_ARRAY candidates are arguably:

              key type    val type    # elems

    "state"   uint32_t    uint32_t    3
    "strtab"  uint32_t    N / A       1
    "gvars"   uint32_t    N / A       1

The "state" map is an odd choice architecturally, but it has a lot of
spare room.

The "gvars" map is arguably a decent choice;  we can think of there being a
big "global variable" of rather arbitrary size that we reference by pointer.

The "strtab" map, for completeness, is another choice.

Let us use gvars.

For code generation, we will need to know the offset of this region.
This patch supports deferring the specification of this offset until
relocation, but I suspect we might actually know the offset at cg time
since it's only dependent on dt_idhash_datasize(dtp->dt_globals).

Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
---
 libdtrace/dt_bpf.c   | 5 +++--
 libdtrace/dt_bpf.h   | 1 +
 libdtrace/dt_cc.c    | 3 +++
 libdtrace/dt_cg.c    | 2 +-
 libdtrace/dt_dlibs.c | 1 +
 libdtrace/dt_impl.h  | 2 ++
 6 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/libdtrace/dt_bpf.c b/libdtrace/dt_bpf.c
index 87cc0814..ee9aa846 100644
--- a/libdtrace/dt_bpf.c
+++ b/libdtrace/dt_bpf.c
@@ -351,8 +351,9 @@ dt_bpf_gmap_create(dtrace_hdl_t *dtp)
 	if (pr_mapfd == -1)
 		return -1;		/* dt_errno is set for us */
 
-	/* global variables */
-	sz = P2ROUNDUP(dt_idhash_datasize(dtp->dt_globals), 8);
+	/* global variables, including a region of zeroes */
+	dtp->dt_zerooffset = P2ROUNDUP(dt_idhash_datasize(dtp->dt_globals), 8);
+	sz = dtp->dt_zerooffset + dtp->dt_zerosize;
 	if (sz > 0 &&
 	    create_gmap(dtp, "gvars", BPF_MAP_TYPE_ARRAY,
 			sizeof(uint32_t), sz, 1) == -1)
diff --git a/libdtrace/dt_bpf.h b/libdtrace/dt_bpf.h
index 12b9fce3..40cbcf43 100644
--- a/libdtrace/dt_bpf.h
+++ b/libdtrace/dt_bpf.h
@@ -36,6 +36,7 @@ extern "C" {
 #define DT_CONST_TASK_COMM	16
 #define DT_CONST_MUTEX_OWNER	17
 #define DT_CONST_RWLOCK_CNTS	18
+#define DT_CONST_ZERO_OFF	19
 
 extern int perf_event_open(struct perf_event_attr *attr, pid_t pid, int cpu,
 			   int group_fd, unsigned long flags);
diff --git a/libdtrace/dt_cc.c b/libdtrace/dt_cc.c
index e3166647..5d85eb82 100644
--- a/libdtrace/dt_cc.c
+++ b/libdtrace/dt_cc.c
@@ -2454,6 +2454,9 @@ dt_link_construct(dtrace_hdl_t *dtp, const dt_probe_t *prp, dtrace_difo_t *dp,
 				nrp->dofr_data = total_offset;
 				continue;
 			}
+			case DT_CONST_ZERO_OFF:
+				nrp->dofr_data = dtp->dt_zerooffset;
+				continue;
 			default:
 				/* probe name -> value is probe id */
 				if (strchr(idp->di_name, ':') != NULL)
diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index a84359ea..f48452ee 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -208,7 +208,7 @@ dt_cg_tramp_prologue_act(dt_pcb_t *pcb, dt_activity_t act)
 		DT_CG_STORE_MAP_PTR("scratchmem", DCTX_SCRATCHMEM);
 	if (dt_idhash_datasize(dtp->dt_aggs) > 0)
 		DT_CG_STORE_MAP_PTR("aggs", DCTX_AGG);
-	if (dt_idhash_datasize(dtp->dt_globals) > 0)
+	if (dt_idhash_datasize(dtp->dt_globals) > 0 || dtp->dt_zerosize > 0)
 		DT_CG_STORE_MAP_PTR("gvars", DCTX_GVARS);
 	if (dtp->dt_maxlvaralloc > 0)
 		DT_CG_STORE_MAP_PTR("lvars", DCTX_LVARS);
diff --git a/libdtrace/dt_dlibs.c b/libdtrace/dt_dlibs.c
index 89ea062d..41ef28d9 100644
--- a/libdtrace/dt_dlibs.c
+++ b/libdtrace/dt_dlibs.c
@@ -89,6 +89,7 @@ static const dt_ident_t		dt_bpf_symbols[] = {
 	DT_BPF_SYMBOL_ID(TASK_COMM, DT_IDENT_SCALAR, DT_CONST_TASK_COMM),
 	DT_BPF_SYMBOL_ID(MUTEX_OWNER, DT_IDENT_SCALAR, DT_CONST_MUTEX_OWNER),
 	DT_BPF_SYMBOL_ID(RWLOCK_CNTS, DT_IDENT_SCALAR, DT_CONST_RWLOCK_CNTS),
+	DT_BPF_SYMBOL_ID(ZERO_OFF, DT_IDENT_SCALAR, DT_CONST_ZERO_OFF),
 
 	/* End-of-list marker */
 	{ NULL, }
diff --git a/libdtrace/dt_impl.h b/libdtrace/dt_impl.h
index 37088a5d..149f8b18 100644
--- a/libdtrace/dt_impl.h
+++ b/libdtrace/dt_impl.h
@@ -295,6 +295,8 @@ struct dtrace_hdl {
 	uint_t dt_maxdvarsize;	/* largest dynamic variable across programs */
 	uint_t dt_maxtuplesize;	/* largest tuple across programs */
 	uint_t dt_maxlvaralloc;	/* largest lvar alloc across pcbs */
+	uint_t dt_zerosize;	/* zero region, size */
+	uint_t dt_zerooffset;	/* zero region, offset */
 	dt_tstring_t *dt_tstrings; /* temporary string slots */
 	dt_list_t dt_modlist;	/* linked list of dt_module_t's */
 	dt_htab_t *dt_mods;	/* hash table of dt_module_t's */
-- 
2.18.4